Feels-like temperature calculator

Feels-like temperature combines temperature, humidity and wind to estimate what the body actually perceives. A thermometer alone does not tell the whole story.

These are two different formulas, not one

"Feels like" is a single name for two opposite phenomena, each with its own equation:

• In heat — the heat index. It depends on humidity. The more humid the air, the less sweat evaporates, and evaporating sweat is the body's main cooling mechanism. At 86 °F with 80% humidity, it feels like about 100 °F.
• In cold — wind chill. It depends on wind, which strips away the thin layer of warmed air against the skin and accelerates heat loss.

That is why high humidity changes nothing in the cold, and strong wind barely alters how a hot, still day feels.

The effect of humidity is not linear

At 86 °F, raising humidity from 40% to 80% adds 15 degrees. At 95 °F, the same change adds 35 degrees. The effect explodes with temperature.

Shade and sun

These values assume shade and light wind. In full sun, add up to 15 °F — which turns an "extreme caution" day into a "danger" one without the forecast number changing at all.